The present invention relates to method for driving a ferroelectric liquid crystal displays, and more particularly to a driving method of a ferroelectric liquid crystal displays in a 1-frame reset mode, using a super twisted nematic (STN) driving IC, and a bias voltage circuit for generating a bias voltage supplied to the STN driving IC.
Ferroelectric liquid crystal displays which can present an image by a simple matrix driving without using active elements, have a characteristic that alignment of the liquid crystal is stored regardless of the interruption of the supplied power, so that contrast is not degraded even though duty is decreased. Also, while the switching of a nematic liquid crystal is carried out by a weak interaction (.DELTA..epsilon..multidot.E .sup.2 /2) between the dielectric anisotrophy (.DELTA..epsilon.) of the liquid crystal and the external electric field (E), the switching of a ferroelectric liquid crystal is carried out by a strong interaction (Ps.multidot.E) between the spontaneous polarization (Ps) of the liquid crystal and the external electric field. Accordingly, the response speed of the ferroelectric liquid crystal becomes to be measured in terms of microseconds, which is much faster than that of the nematic liquid crystal. Here, the basic characteristics of the ferroelectric liquid crystal display, which should be considered in the driving thereof, are as follows.
Generally, if a sustained DC component is applied to the liquid crystal, the liquid crystal deteriorates due to the electrochemical reaction. Also, the alignment orientation of the ferroelectric liquid crystal is changed due to the polarity of pulses. Therefore, a waveform of one period supplied to the liquid crystal during driving must have no DC component, and the data is displayed by selectively supplying one of both pulses of opposite polarities. Additionally, the pulse width supplied to the ferroelectric liquid crystal is restricted by the kind of liquid crystal, so that, when the pulse width is wide, the threshold voltage which causes state transition becomes low. In other words, the value obtained by multiplying threshold voltage V.sub.th by pulse width .tau. is a generally constant, and thus the pulse width must be lengthened to lower the driving voltage. However, to lengthen the pulse width undesirably requires a long period of time for expressing one pixel.
Meanwhile, the driving method of a ferroelectric liquid crystal display is classified into 5-pulse, 4-pulse, 3-pulse and 2-pulse techniques according to the number of the pulses required to display one pixel. Here, the 4-pulse and 2-pulse techniques are termed the 2-field method and 1-frame reset method, respectively, and will be described in detail with reference to FIGS. 1 and 2.
FIG. 1 illustrates a conventional 2-field method for displaying one picture by performing scanning twice, i.e., two fields wherein a first data state is designated in the first field, and a second data state is designated in the second field. However, in such a driving method, since the time required for expressing one picture becomes twice the field time, the displayed number of pictures in a unit time is halved, which impedes presentation of smoothly succeeding pictures. Furthermore, four pulses are required for displaying one pixel, which in turn narrows the pulse width, and raises the driving voltage in case of displaying many pixels within the unit time.
FIG. 2 illustrates 1-frame reset method, wherein one picture can be expressed by scanning once, and so that two pulses are required for expressing one pixel. Therefore, a larger number of pixels can be driven within a unit time. The voltage supplied to each electrode is one of three voltages. These features consequently simplify driving. However, an exclusive driving IC for driving the ferroelectric liquid crystal should be necessarily developed to realize the driving. That is, driving by way of the conventional STN driving IC becomes very complicated and is, for all practical purposes, impossible.